Compositions of dispersible organosilicone copolymers are obtainable by emulsion polymerization in an aqueous medium, the stabilization of the resulting dispersions further requiring emulsifiers or protective colloids, as described, for example, in EP-A 1308468 or EP-A 771826.
EP-A 614924 as well describes the preparation of organosilicone copolymers by means of emulsion polymerization in water with the addition of surface-active substances. The process described therein is applicable only to free-radically polymerizable silicone monomers of low molecular weight, since, if high molecular weight silicone monomers are employed, there is a reduction in the rate of polymerization and, furthermore, there is aggregation of the polymers during the polymerization, or a decrease in the stability of the copolymer emulsion.
EP-A 352339 describes a process for preparing organo-silicone copolymers by means of solution polymerization, with the silicone fraction being included in the initial charge in the solvent, and with continuous metering of a mixture of monomers and oil-soluble initiator. The copolymers obtainable in this way, however, are not dispersible in water. Dispersing these copolymers requires dispersing assistants such as emulsifiers or protective colloids.
In EP-A 810243 and JP-A 05-009248, silicone macromers are polymerized with organic monomers in aqueous emulsion, operating exclusively with oil-soluble initiator. A disadvantage with processes involving initiation with oil-soluble initiators is the inadequate stability of the resultant dispersions, which have a very strong propensity toward phase separation.
From U.S. Pat. No. 5,618,879 an emulsion copolymerization of a mixture of silicone macromer and ethylenically unsaturated monomer in the presence of anionic emulsifiers and water-soluble initiator in water is described. In JP-A 05-140255 the silicone macromer is dissolved in the organic monomer, emulsification takes place in water, using anionic emulsifier, and the polymerization is initiated using water-soluble initiator. Disadvantageous features in this case too are that surface-active substances must be used and that a considerable fraction—more than 20%—of the silicone macromer is not copolymerized.
A common feature of the known compositions of dispersible organosilicone copolymers is that their preparation by copolymerization of silicone macromers with organic monomers requires the presence of emulsifiers or protective colloids. The organosilicone copolymer compositions obtainable in this way, however, have a propensity toward phase separation. Phase separation during the polymerization leads to cloudy polymer films. Migration of the emulsifiers or protective colloids in organosilicone copolymer compositions is known to have an adverse effect on the water resistance, adhesion or stability properties of the organosilicone copolymer compositions.
A further problem lies in the provision of highly transparent dispersible organosilicone copolymer compositions with a high silicone fraction. Particularly when organosilicone copolymers with a silicone fraction of more than 20% by weight are prepared, poor compatibility between olefinic monomers and silicones leads, during the free-radical polymerization, to problems due to phase separation or gelling, which results in the organosilicone copolymers becoming cloudy.